title: Evolution of the pair rule gene network: Insights from a centipede.
abstract: Comparative studies have examined the expression and function of homologues of the Drosophila melanogaster pair rule and segment polarity genes in a range of arthropods. The segment polarity gene homologues have a conserved role in the specification of the parasegment boundary, but the degree of conservation of the upstream patterning genes has proved more variable. Using genomic resources we identify a complete set of pair rule gene homologues from the centipede Strigamia maritima, and document a detailed time series of expression during trunk segmentation. We find supportive evidence for a conserved hierarchical organisation of the pair rule genes, with a division into early- and late-activated genes which parallels the functional division into primary and secondary pair rule genes described in insects. We confirm that the relative expression of sloppy-paired and paired with respect to wingless and engrailed at the parasegment boundary is conserved between myriapods and insects; suggesting that functional interactions between these genes might be an ancient feature of arthropod segment patterning. However, we find that the relative expression of a number of the primary pair rule genes is divergent between myriapods and insects. This corroborates suggestions that the evolution of upper tiers in the segmentation gene network is more flexible. Finally, we find that the expression of the Strigamia pair rule genes in periodic patterns is restricted to the ectoderm. This suggests that any direct role of these genes in segmentation is restricted to this germ layer, and that mesoderm segmentation is either dependent on the ectoderm, or occurs through an independent mechanism.
authors.affiliation: Laboratory for Development and Evolution, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB23EJ, UK. Electronic address: jg457@cam.ac.uk.
authors.forename: Jack
authors.initials: J
authors.lastname: Green
authors.forename: Michael
authors.initials: M
authors.lastname: Akam
mesh_terms: D000818:Animals; D000845:Annelida / Q000196:embryology* / Q000235:genetics*; D019521:Body Patterning / Q000235:genetics; D017124:Conserved Sequence; D004475:Ectoderm / Q000196:embryology / Q000378:metabolism; D004625:Embryo, Nonmammalian / Q000378:metabolism; D019143:Evolution, Molecular; D018507:Gene Expression Regulation, Developmental; D053263:Gene Regulatory Networks; D005796:Genes; D012689:Sequence Homology, Nucleic Acid; D013997:Time Factors
keywords: Arthropod; Chilopod; Evolution; Pair rule gene; Pattern formation; Segmentation